A network card (WLAN card) is used in a computer for mass transmission of data within a network (such as standard LAN protocol) so as to eradicate the problem of forming complicated wired cables (line connection) for coupling pluralities of computers together in the network. In fact, in a WLAN, antenna devices replace the wired cables with the assistance of the network card for conducting data transmission. A monopole antenna assembly has a compact size and can be formed on a printed circuit board by screen-printing technology. Due to its compact size and lightness in weight, the monopole antenna assembly is employed in a wireless communication product (such a cellular phone) for signal transmission.
FIG. 1 shows a conventional monopole antenna assembly to include a baseboard 10, a grounding metal layer 12, and a radiating metal strip 14. The radiating metal strip 14 is mounted on the upper surface of the baseboard 10 by the screen printing technology, and includes a feeding strip section 16 with a feeding pin 18 for coupling to a matching circuit (not shown). The grounding metal layer 14 is mounted on the lower surface of the baseboard 10, and is spaced apart from the feeding pin 18 by a quarter wavelength of the intended transmission frequency band.
It is noted that the dimension of the radiating metal strip 14 cannot be reduced further since it is generally limited to be within the range of the quarter wavelength of the transmission frequency band. However, the passivation components on the integrated circuit are in the trend to reduce the size, limitation of the monopole antenna assembly within the quarter wavelength blocks the research for reducing the dimension of the conventional antenna assembly.
In addition, the aforesaid monopole antenna assembly is only compatible with a single channel, such as a wireless local area network. The operating band is ISM 2.4 GHz, which is presently assigned to Industrial Scientific Medical band. Since the use of wireless technology (like Bluetooth) is more and more common day-by-day, the telecommunication apparatuses may suffer co-channel interference or next-channel interference. Moreover, the high frequency bandwidth ranging 8-9 GHz is not compatible with the presently available protocols and is not allowed for commercial use today. The larger the size of the radiating metal strip, relative to the operating wavelength, the greater antennas gain, i.e. large, flatter conductors have less resistive lose due to their large area surface. However, it is absolutely impossible to design large antenna assembly within the telecommunication apparatuses.